Changes in tissue stiffness have long been associated with disease. Traditionally, palpation is one of the primary methods of detecting and characterizing tissue pathologies. It is well known that a hard mass within an organ is often a sign of an abnormality. Several diagnostic imaging techniques have recently been developed to provide for non-invasive characterization of tissue stiffness.
One measure of tissue stiffness is a physical quantity called Young's modulus, which is typically expressed in units of Pascals, or more commonly kilo Pascals (kPa). If an external uniform compression (or stress, S) is applied to a solid tissue and this induces a deformation (or strain, e) of the tissue, Young's modulus is defined simply as the ratio between applied stress and the induced strain:E=S/e. 
Hard tissues have a higher Young's modulus than soft tissues. Being able to measure the Young's modulus of a tissue helps a physician in differentiating between benign and malignant tumors, detecting liver fibrosis and cirrhosis, detecting prostate cancer lesions, etc.
A collection of diagnostic and imaging modalities and processing techniques have been developed to allow clinicians to evaluate tissue stiffness using ultrasonography. These techniques are collectively referred to herein as Elastography. In addition to providing information about tissue stiffness, some elastography techniques may also be used to reveal other stiffness properties of tissue, such as axial strain, lateral strain, Poisson's Ratio, and other common strain and strain-related parameters. Any of these or other strain-related parameters may be displayed in shaded grayscale or color displays to provide visual representations of such strain-related parameters. Such information may be displayed in relation to two or three dimensional data.
Elastography techniques may be broadly divided into two categories, “quasi-static elastography” techniques and “dynamic elastography” techniques.
In quasi-static elastography, tissue strain is induced by mechanical compression of a tissue region of interest, such as by pressing against a tissue with a probe a hand or other device. In other cases, strain may be induced by compression caused by muscular action or the movement of adjacent organs. Images of the tissue region of interest are then obtained in two (or more) quasi-static states, for example, no compression and a given positive compression. Strain may be deduced from these two images by computing gradients of the relative local shifts or displacements in the images along the compression axis. Quasi-static elastography is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount the tissue yields under this pressure.
In dynamic elastography, a low-frequency vibration is applied to the tissue and the speed of resulting tissue vibrations is detected. Because the speed of the resulting low-frequency wave is related to the stiffness of the tissue in which it travels, the stiffness of a tissue may be approximated from wave propagation speed.
Many existing dynamic elastography techniques use ultrasound Doppler imaging methods to detect the speed of the propagating vibrations. However, inherent limitations in standard Doppler imaging present substantial challenges when attempting to measure the desired propagation speed. This is at least partly because the waves of most interest tend to have a significant propagation component in a direction perpendicular to the direction of the initial low-frequency vibration.
As used herein, the term dynamic elastography may include a wide range of techniques, including Acoustic Radiation Force Impulse imaging (ARFI); Virtual Touch Tissue Imaging; Shearwave Dispersion Ultrasound Vibrometry (SDUV); Harmonic Motion Imaging (HMI); Supersonic Shear Imaging (SSI); Spatially Modulated Ultrasound Radiation Force (SMURF) imaging.